The “it” of “It Comes at Night” doesn’t creep about on four legs or slither on its belly. No fangs or claws. No growls or shrieks.

The subject of Trey Edward Shults’ sophomore feature (after last year’s devastating family drama “Krisha”) is fear. Fear of both the unknown, of whatever may be lying in wait for us, and fear of our own human selves which, given the right circumstances, can devolve into monsters far scarier than those lurking in the imagination.

As the film opens an old man is dying. His eyes are black. Festering pustules dot his body. Blood seeps from his nose and mouth. He breathes in gasps.

Whatever is killing the old man has spooked the other members of his family, who say their muffled goodbyes through biohazard masks. Then they load him up in a wheelbarrow and push him out to a pit where he will be dispatched with one bullet and his remains burned.

This is the new normal for Paul (Joel Egerton), his wife Sarah (Carmen Ejogo), and their teenage son Travis (Kelvin Harrison Jr.). They live in a house deep in the woods. The windows are boarded up so that from the outside the place looks abandoned. They don’t venture outside any more than is absolutely necessary. They are on constant alert for unwanted visitors.

What catastrophe has befallen mankind that they must live this way? Schults’ screenplay never provides an answer and, anyway, that’s not what “It Comes at Night” is about.

Late one night the three hear someone trying to break in. They capture the intruder, a young man named Will (Christopher Abbott) who claims he thought the house was empty when he began scavenging for supplies. Will says his wife and young son are waiting for him in a cabin nearly 50 miles away.